Residual stress is internal stress in a material that is present even if no any external force is applied. Residual stress can be of tensile or compressive or mixed depending on the treatment under which the material has passed.
Different processes like Rolling, Forging, Drawing, Tubing produces different stresses on surface and inside bulk.
During Rolling: Compressive stress is generated on surface and tensile at the centre
During Forging: Residual stress is generated due to quenching after forging. Hot forging produces very less residual stress.
During Drawing: For reduction less than 1 %
Compressive stress at the outer surface, tensile at the centre.
For reduction more than 1%
Tensile at the outer surface, compressive at the centre
During Tubing: Tensile at the outer surface and compressive at the inner surface of tube.
Please watch video to understand more on residual stresses.
Measurement of this residual stress is done using Hole Drill Method.
In hole drill method, a strain gauge is placed at the the surface and using drilling mechanisms, a drill is made at the strain gauge and when drill is taken out after drill certain depth then due to residual stresses there is contraction or expansion in the strain gauge and that produces electrical signal. It is converted into strain value and further using standard equation, strain is converted into stress and we get a residual stress measurement.
Please watch video to know more about the measurement technique.